The present invention generally relates to methods and systems for producing a pulp from lignocellulosic material, such as wood chips, using chemical cooking techniques. The pulp may be produced in a continuous flow chemical digester vessel.
Lignocellulosic material, such as wood, is conventionally comminuted into wood chips before being cooked in a digester vessel, such as a continuous or batch vessel. The size of the wood chips has primarily been set to enhance digester performance and, particularly, to avoid plugging the bottom of the digester vessel with collapsed chips.
Softwood chips are typically cooked to Kappa numbers of 20 to 33 and hardwood chips are cooked to a Kappa numbers of 15 to 20. The Kappa number indicates the residual lignin content of wood pulp. At these conventional Kappa numbers, thin chips, e.g., chips having a thickness of less than 7 mm, become soft and easily compressed at the bottom of the digester vessel.
Compressed, soft thin chips become densely packed at the bottom of the digester vessel, plug the bottom and impede the flow of wash liquor through the chips in the wash zone of the digester vessel. When compression of soft chips is severe, the bottom of the digester becomes choked with chips such that insufficient liquid flows through the chips in the column of chips in the vessel. Under such conditions, a mass transfer problem can arise in which the chips no longer move uniformly downward to the chip discharge outlet at the bottom of the vessel.
Compressed soft, thin chips can form chip agglomerations that plug and block chip flow down through the lower portion of a digester vessel. Channels may form in the thin chip agglomerations that allow some chips to flow to the bottom of the vessel while other chips are bound in the agglomeration. The channels are not desired as they are inconsistent with uniform chip flow down the digester vessel and allow the agglomerations of chips to remain in the vessel for extended periods.
Chip agglomerations may inhibit wash liquids intended to flow through the chips to remove used or spent cooking liquor (black liquor) and lignin prior to exiting the chips/pulp exiting the digester. An agglomeration of cooked thin chips at the bottom of a digester vessel can inhibit the removal of black liquor before the chips are discharged from the digester vessel. An agglomeration of cooked thin chips may also plug or block the screens in the sidewalls of the digester vessel.
A high content of very thin chips and pin chips (collectively referred to as “small chips”) can cause problems in an upper portion of a digester vessel. Small chips may plug the screens in the upper regions of the digester vessel. Plugged screens prevent the extraction of liquor from the upper portion of the digester vessel.
When the vessel becomes excessively compacted, the continuous cooking operation is temporarily stopped and cold liquor added to the bottom of the digester to cool down, break up and remove the agglomeration of chips. The lower pulp production rate or temporary halt to chip production results in a reduction in the pulp production realized by the mill and higher maintenance costs.
Due to the difficulty in processing thin and small chips, an average chip thickness of 8 mm is a standard minimum sized chip to be formed at a mill for use in a continuous digester vessel. When the average chip length is 22 mm (millimeter) to 30 mm in length, the thickness of the chips is generally less than 8 mm, with 85% to 90% of the chips having a thickness in a range of 8 mm to 2 mm.
Chip screens in the chip feed system are commonly used to select chips having an acceptable thickness. The screens may be positioned at an inlet to the chip bin for the digester vessel. A chip screen may have first screen of 8 mm slots and second screen having 7 mm diameter holes. Chips are selected as those that pass through the first screen and are retained by the second screen. Screening chips is a technique for classifying the chips. Chip classification is commonly done according to a SCAN-CM 40:01 chip size distribution analyzing method. According to this method, acceptable chips for continuous digesting are those that pass through an 8 mm slot and are retained on a tray with 7 mm holes.
Conventional wisdom is that large amounts of thin and small chips should not be processed in a conventional continuous digester vessel. Thin and small chips, such as pin chips and sawdust, are conventionally processed in a Pandia Digester offered by GL&V, Bauer M&D digesters and Metso pin chip processes, or a specially adapted Kamyr® digester.
To avoid the problems associated with small and thin chips, the conventional wisdom has been that chips for a conventional continuous digester should be sufficiently large, e.g., average chip thickness of 8 m and lengths 25 to 30 mm for softwood and 22 to 24 mm for hardwood, to avoid excessive softening the chips in the digester vessel.